The purpose of the study was to analyse changes in professionally-defined need and self-perceived need between the ages of 12-20 years. In 1985, occlusion and perceived need for treatment were recorded in 306 12-year-olds (born 1973) living in five communes in Finnmark, the most northern part of Norway. In 1993-94, 271 of the individuals were traced through the Norwegian Central Person Register and invited in letters to attend a clinical examination and an interview. A total of 80 individuals responded, with a mean age of 20.7 years. Professionally-defined need was assessed according to the Need for Orthodontic Treatment Index (NOTI) which is used by the Norwegian Health Insurance System for reimbursement of treatment costs. Change in category of need between 12-20 years was used as a measurement of health gain. Perceived need was measured by the individuals' responses to a question about desire for treatment. Individuals who received treatment (n = 22) showed an improvement in occlusion (health gain) corresponding on average to one category of need. The occlusal status of the untreated individuals was unchanged. All the individuals that desired treatment at 12 years of age (n = 15) did not want treatment at 20, and they were satisfied with their dental arrangement. Most of these individuals had received treatment, but a few with minor malocclusions had at age 12 been informed about the small objective need and refrained from treatment. All the individuals that wanted treatment at 20 years of age, 29 per cent of the total sample, had not desired treatment at 12 years of age. The results underline the importance of information and education during orthodontic counselling of potential patients.